The motile tips of embryonic axons integrate signaling information from a multitude of guidance molecules during development, plasticity, and regeneration following injury. The rodent taste system provides an especially attractive model in which to study these processes. Perhaps the earliest event in taste nerve development is the sorting of geniculate axons into one of two nerves that diverge from one another at a right angle as they exit the ganglion. No differences between these axons have been described. Cell- contact dependent processes have a prominent role in pathway and target discrimination, and could underlie the divergent signaling. Eph receptors and ephrins are implicated in contact-dependent axon divergence and target selection elsewhere both within the nervous system and in other tissues, but neither their expression nor their function has been explored in the taste system. Our preliminary data indicate that several members are expressed in the taste system. Approaches: (Aim 1) We will test the hypothesis that Eph/ephrins are differentially expressed among early embryonic sensory neurons that ultimately innervate well separated targets. To this end, nerve labeling will be carried out, and then cells from each of these populations will be microdissected and subjected to antisense RNA amplification and PCR, enabling us to profile up to 10 Eph/ephrin family members per cell. (Aim 2) We will perturb Eph/ephrin function in organ cultured embryos. We will place beads soaked in diffusible (Fc) constructs of these molecules near the ganglion to test their impact on nerve divergence. [unreadable] [unreadable] [unreadable]